Viacom has mounted the biggest challenge yet to the Internet's most popular and wildly successful Web sites, filing a $1 billion lawsuit against YouTube and its parent company Google that could have far-reaching effects on online commerce.

Viacom, owner of MTV and Comedy Central, among others, charges in a suit filed Tuesday that more than 160,000 clips, including snippets of "The Colbert Report" and "South Park," appear on YouTube and have been seen 1.5 billion times. It is asking the court to stop YouTube, acquired last year by Google for $1.65 billion, from publishing the videos on the site without its authorization.

"I think it's significant that a real large player has decided to go after Google," said Lee Bromberg, an intellectual property lawyer at Bromberg & Sunstein LLP in Boston. "It could mean real trouble for the Google YouTube model."

The lawsuit represents a critical step in the evolution of digital media as the growing online video phenomenon attracts millions of fans. New technologies have made it easier than ever for people to share their favorite television shows and movies, as well as the videos they've made themselves, on the Internet.

Also at stake is billions of dollars in advertising revenue. With advertising budgets shifting from traditional media to the Internet, Google and YouTube -- not to mention media companies such as Viacom -- have been trying to find new ways to capitalize on the viewers flocking to the Web to watch video.

Until now, technology and media companies have sought to find common ground. YouTube signed revenue-sharing agreements with such media owners as CBS and the National Basketball Association -- and had been in negotiations with Viacom -- to post authorized clips online. Media companies also have made their television shows and movie clips available online through sites such as Joost, ABC.com and Apple's iTunes online store.

But talks between Viacom and YouTube fell apart earlier this year. And now Viacom has accused YouTube of dragging its feet in dealing with copyright issues at the same time it makes money off videos created and funded by Viacom.

"There is no question that YouTube and Google are continuing to take the fruit of our efforts without permission," Viacom said in a statement. "Therefore we must turn to the courts to prevent Google and YouTube from continuing to steal value from artists and to obtain compensation for the significant damage they have caused."

Copyright concerns have plagued YouTube and many of its competitors almost since their inception.

But YouTube has argued that its users -- usually fans of the show -- have helped build buzz, generating increased interest for shows such as "Saturday Night Live." Indeed, CBS has said that putting its clips on YouTube has increased viewership for the "Late Show with David Letterman."

Under current laws, YouTube must remove the unauthorized clips once the owner has identified and alerted the company about them. But the problem persists because even as YouTube removes one clip, another is uploaded by a different user.

Last month, Viacom ordered YouTube to take down more than 100,000 clips. YouTube said it complied -- and indeed, a search for certain shows turns up notes that the clips have been removed because they violated copyright rules.

In a statement, Google said that it is "confident that YouTube has respected the legal rights of copyright holders and believe the courts will agree."

Alexander Macgillivray, Google's associate general counsel for products and intellectual property, also said that YouTube has one of the fastest response times in taking down unauthorized clips. "We are going to continue doing what we're doing," he said about running YouTube.

YouTube has also been developing software to find and remove copyrighted clips. But Viacom said in its 27-page lawsuit, filed in U.S. District Court for the Southern District of New York, that YouTube has "deliberately withheld" the tools to force Viacom to agree to lesser financial terms.

Some legal experts believe Viacom's lawsuit could be a negotiating tactic that eventually will lead to a settlement. But if it goes to court, it could cause a new interpretation of current copyright law -- that is, the Digital Millennium Copyright Act.

"It's going to present some new questions to the court: how to apply the DMCA, which clearly wasn't written with this in mind," said Michael Graif, an attorney who specializes in intellectual property law for Venable LLP in New York.

"Does the law really require Viacom to send hundreds of thousands of take-down notices?" Graif said. "That seems to be pretty burdensome, especially when they are the one being harmed. ... Viacom finally threw up their hands."

YouTube's popularity has yet to wane despite its legal troubles. Hitwise, an Internet market research company, found that YouTube's audience increased even after Viacom demanded that its clips be deleted from the site last month. Hitwise also discovered that some of the most popular clips were not copyrighted but were user-generated ones, such as a short animated video on "Charlie the Unicorn."

Nevertheless, Viacom's clips tend to be widely watched on YouTube -- "The Colbert Report" and "The Daily Show with Jon Stewart" for instance, have regularly appeared under "most viewed" or "top rated." Their format, with each episode broken up by different themes and guest appearances, also fits the current culture of watching short clips rather than long, extended shows.